1. Field of the Invention
The present invention relates to a semiconductor device that generates and uses an internal clock signal in accordance with a clock signal that is input from the outside.
2. Description of the Related Art
The conventional semiconductor device uses a clock signal input from the outside as an internal clock signal without being processed. Therefore, when the duty ratio of the external input signal is 50%, the duty ratio of the internal clock signal is also 50%. However, as the signal speed becomes faster, there is a possibility that the duty ratio of the external input signal varies from 50%. Nevertheless, it is required that the semiconductor device operates normally in such a case. Therefore, JP-1998-144074A and JP-2004-206879A disclose various semiconductor devices for ensuring room for variation in the clock signal that is input from the outside.
There is a need for a semiconductor device that can normally operate even if the duty ratio of the clock signal, that is input, varies, like this. Therefore, when the operation test for the semiconductor device is performed, it is necessary to perform the test during the time when the duty ratio of the clock test that is input varies.
However, in the conventional semiconductor device, when a simplified measurement device having no function for changing the duty ratio of the clock signal that is generated is used, it is impossible to change the duty ratio of the clock signal that is input from the outside to the semiconductor device and to perform the operation test.
Under this situation, for example, in a module provided with a waveform shaping function, such as PLL (Phase-Locked Loop), the conventional semiconductor device, while changing the duty ratio, cannot test the operation margin of the PLL and the operation margin of an asynchronous circuit.
The conventional semiconductor device has a problem that the operation margin during the time when, while the duty ratio varies, cannot be tested, when not using a measurement device that can change the duty ratio of the clock signal that is supplied.